1. Field of the Invention
The present invention is related to signal encryption and decryption, and more particularly, to a method and apparatus for uniquely encrypting a plurality of services at a transmission site in a multi-service communications system and for decrypting the services at a remote reception site. The present invention reduces the amount of encryption related information that must be transmitted to the reception site for decryption purposes.
2. Background of the Invention
Although the background of the invention is described herein in the context of subscription television systems, the present invention is by no means limited thereto. Rather, the present invention may be employed in any multi-service communications system in which it is desirable to uniquely encrypt a plurality of individual services.
Preventing unauthorized access to transmitted television signals is of paramount concern to the subscription television industry. In the subscription television industry, "programmers" provide "services" for distribution to various reception sites. A "service" is a signal that contains some type of information, such as video, audio, closed-captioning or teletext information. A single programmer may wish to supply many services. Typically, a programmer will supply various services via satellite to cable television operators who, in turn, will provide the services to cable subscribers. Alternatively, a programmer may supply services directly to direct broadcast satellite (DBS) subscribers. Techniques for preventing unauthorized access to these services have become increasingly important. Cryptographic techniques, widely used in the computer security field, are ideal for preventing unauthorized access to services in a subscription television system. One form of cryptography, known as "private key" cryptography, is particularly well suited for such purposes. With private key cryptography, the transmitter and receiver share a common encryption key (sometimes also referred to as a "seed") that is used for both encrypting and decrypting. An exemplary cryptographic system 10 employing private key cryptography is depicted in FIG. 1. As shown, a service to be transmitted is provided to an encryptor 12. A seed generator 14 generates a unique encryption seed which is fed to the encryptor 12. Encryptor 12 encrypts the service in accordance with an encryption algorithm upon the encryption algorithm being "keyed" by the encryption seed. Encryption seeds are typically binary values having lengths that may range from 8 to 56 bits. Both the encryption seed and the encrypted service data are provided to a multiplexer 11 that multiplexes the encryption seed and encrypted service data and provides the multiplexed data signal to a transmitter 16. Transmitter 16 transmits the multiplexed data signal to a remote reception site. A receiver 18 at the reception site receives the multiplexed data signal and a demultiplexer 19 separates the encryption seed from the encrypted service data. The encrypted service data and encryption seed are then provided to a decryptor 20. Decryptor 20 employs the inverse of the encryption algorithm employed by encryptor 12, and therefore, upon "keying" the encryption algorithm with the received encryption seed, the decryptor 20 is able to decrypt the received service data.
Each programmer in a subscription television system typically provides many different services. For various reasons, programmers prefer to uniquely encrypt each service. Accordingly, a unique encryption seed must be generated for each service, and each unique encryption seed must somehow be provided to the reception site. U.S. Pat. Nos. 5,029,207 (Gammie), 4,613,901 (Gilhousen et al.), 4,736,422 (Mason) and 4,802,215 (Mason) each describe systems in which the encryption seeds used to encrypt each service are transmitted to a reception site along with the encrypted service data. For security reasons, the encryption seeds themselves are often encrypted prior to transmission. Co-pending application Ser. No. 07/984,461, which is cross-referenced above, describes a method and apparatus having particular applicability to the encryption of seeds at each of a plurality of programmer sites.
Additional security is typically provided by periodically changing the encryption seeds for each service at the transmission site. Frequently changing the encryption seeds hampers piracy efforts. However, because each new encryption seed must be transmitted to the reception site, bandwidth that would otherwise be available for service data is consumed. Moreover, the encryption seeds are often heavily error protected which results in even greater bandwidth consumption.
When the number of services being provided to subscribers is small, the bandwidth consumed by the transmission of encryption seeds is not a problem. With the advent of various compression technologies, however, the subscription television industry is experiencing a vast increase in the number of individual services that can be provided to subscribers. As the number of available services increases, the additional bandwidth needed for the transmission of encryption seeds becomes prohibitive. Consequently, there is a need for encryption methods and apparatus that reduce the amount of encryption related information that must be transmitted to a reception site, without sacrificing cryptographic strength. The present invention satisfied this need.